Born in Dingwall, Scotland, he was educated at King’s College, Aberdeen University. A cousin of George Simpson, he came to Norway House in 1829 as George’s secretary. In 1836 he was appointed by the Hudson's Bay Company as co-leader of an arctic expedition, with Peter Warren Dease. Over the winter of 1836-37 he snowshoed from Fort Garry to Fort Chipewyan on Lake Athabasca, a distance of 1,277 miles in 46 marching days. In 1837 the expedition explored the arctic coastline west of the Mackenzie River, then in 1838 journeyed down the Coppermine River. In December 1839 Simpson set out for Red River from the Boothia Peninsula, making it back over the 1,800 miles in 61 days. He had demonstrated that he was very fit, but over the years of the expedition also that he was extremely conceited. He hastened back to England to gain approval for further discoveries, not knowing that the authorization was already on its way. Unwilling to wait for the ships at Hudson Bay, he decided to travel overland by horse through the United States. According to companions with him, he became increasingly anxious and even deranged during the journey, which ended with two colleagues shot dead and Simpson with his head blown off. It seemed to contemporaries a clear case of murder and suicide. His Narrative of the Discoveries on the North Coast of America ... during the years 1836-9 was published by his brother in 1843.